[0001] The present invention relates to a semiconductor laser device having a current blocking
layer.
[0002] A refractive index guided semiconductor laser device supplied with refractive index
difference in a direction parallel to an active layer for forming a light guide is
developed in general. Fig. 34 is a typical sectional view showing a conventional semiconductor
laser device 120 described in Japanese Patent Laying-Open No. 8-222801 (1996).
[0003] In the semiconductor laser device 120 shown in Fig. 34, an n-type cladding layer
122, an active layer 123, a p-type cladding layer 124 and a p-type contact layer 127
are successively formed on an n-type substrate 121, and the p-type contact layer 127
and the p-type cladding layer 124 are etched for forming flat portions on a ridge
portion and on both sides of the ridge portion.
[0004] Further, a first current blocking layer 125 having a low carrier concentration is
formed on the flat portions of the p-type cladding layer 124 located on both sides
of the ridge portion, and an n-type current blocking layer 126 is formed on the first
current blocking layer 125 having a low carrier concentration. A p-type contact layer
128 is formed on the p-type contact layer 127 and the n-type current blocking layer
126.
[0005] When the semiconductor laser device 120 is driven, a reverse bias voltage is applied
to a p-n junction between the n-type current blocking layer 126 and the p-type cladding
layer 124. Thus, the n-type current blocking layer 126 cuts off a current so that
the current is injected into the ridge portion in a narrowed state.
[0006] In general, a p-n junction formed between an n-type current blocking layer and a
p-type cladding layer has large electric capacitance, and hence serves as a factor
inhibiting high-speed operation of a semiconductor laser device. The electric capacitance
of the p-n junction is increased as the carrier concentration in this p-n junction
is increased.
[0007] Therefore, the semiconductor laser device 120 shown in Fig .34 is provided with the
current blocking layer 125 having a low carrier concentration, in order to reduce
the electric capacitance in the p-n junction between the n-type current blocking layer
126 and the p-type cladding layer 124.
[0008] This current blocking layer 125 has a lower carrier concentration than the n-type
current blocking layer 126. Therefore, the current blocking layer 125 having a low
carrier concentration defines a depletion region in the p-n junction between the n-type
current blocking layer 126 and the p-type cladding layer 124, for reducing the electric
capacitance. Thus, the semiconductor laser device 120 is enabled to operate at a high
frequency.
[0009] In the semiconductor laser device 120 having the current blocking layer 125 of a
low carrier concentration having a narrower band gap than the p-type cladding layer
124, however, valence bands of the p-type cladding layer 124 and the current blocking
layer 125 of a low carrier concentration have energy band structures shown in Fig.
35.
[0010] Fig. 35 is a model diagram showing the energy band structures of the valence bands
of the p-type cladding layer 124 and the current blocking layer 125 having a low carrier
concentration. As shown in Fig. 35, the band gap of the current blocking layer 125
having a low carrier concentration is sufficiently smaller than the band gap of the
p-type cladding layer 124, and hence carriers are readily injected from the p-type
cladding layer 124 into the current blocking layer 125 having a low carrier concentration
and stored therein. Consequently, since depletion of the p-n junction between the
n-type current blocking layer 126 and the p-type cladding layer 124 is inhibited,
electric capacitance between the current blocking layer 125 having a low carrier concentration
and the p-type cladding layer 124 is increased. Therefore, the operating speed of
the semiconductor laser device 120 cannot be sufficiently increased.
[0011] An object of the present invention is to provide a semiconductor laser device sufficiently
increased in operating speed.
[0012] A semiconductor laser device according to an aspect of the present invention comprises
an active layer, a first cladding layer of a first conduction type provided on the
active layer, a current blocking layer of a second conduction type provided on the
first cladding layer except a current injection region, a low carrier concentration
layer provided on the side of the current blocking layer between the first cladding
layer and the current blocking layer and having a lower carrier concentration than
the current blocking layer and a depletion enhancement layer provided on the side
of the first cladding layer between the first cladding layer and the current blocking
layer for inhibiting storage of carriers in the low carrier concentration layer.
[0013] In the semiconductor laser device, the depletion enhancement layer inhibits storage
of carriers from the first cladding layer into the low carrier concentration layer.
Thus, the low carrier concentration layer is kept in a depleted state. Therefore,
electric capacitance between the current blocking layer and the first cladding layer
is kept small for sufficiently increasing the operating speed of the semiconductor
laser device.
[0014] The band gaps of the first cladding layer, the depletion enhancement layer and the
low carrier concentration layer may be reduced in this order.
[0015] Thus, the depletion enhancement layer having an intermediate band gap is provided
between the first cladding layer having a large band gap and the low carrier concentration
layer having a small band gap.
[0016] In this case, the band offset between the first cladding layer and the depletion
enhancement layer is smaller than the band offset between the first cladding layer
and the low carrier concentration layer, whereby carriers are hardly injected from
the first cladding layer into the depletion enhancement layer while carriers are more
hardly injected into the low carrier concentration layer. Further, the carriers are
injected from the first cladding layer into both of the low carrier concentration
layer and the depletion enhancement layer in a divided manner, whereby the quantity
of carriers stored in the low carrier concentration layer is reduced. Thus, storage
of carriers in the low carrier concentration layer can be inhibited by the simple
structure of setting the band gap of the depletion enhancement layer to the intermediate
level between the low carrier concentration layer and the first cladding layer.
[0017] The first cladding layer may have a flat portion formed on the active layer and a
ridge portion formed on the flat portion in the current injection region, the depletion
enhancement layer may be formed on the flat portion located on both sides of the ridge
portion and on the side surfaces of the ridge portion, and the low carrier concentration
layer and the current blocking layer may be successively formed on the depletion enhancement
layer.
[0018] In this case, the depletion enhancement layer inhibits storage of carriers from the
flat portion of the first cladding layer into the low carrier concentration layer.
Thus, the low carrier concentration layer is kept in the depleted state, and the electric
capacitance between the flat portion of the first cladding layer and the current blocking
layer is kept small.
[0019] The thickness of the depletion enhancement layer is preferably at least 10 nm. Thus,
the semiconductor laser device is more improved in high-frequency characteristic.
[0020] The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus,
the semiconductor laser device is further improved in high-frequency characteristic.
[0021] The semiconductor laser device may further comprise a ridge-shaped second cladding
layer of a first conduction type provided on the depletion enhancement layer in the
current injection region, the depletion enhancement layer may be formed on the first
cladding layer, and the lower carrier concentration layer and the current blocking
layer may be successively formed on the depletion enhancement layer located on both
sides of the second cladding layer and on the side surfaces of the second cladding
layer.
[0022] In this case, the depletion enhancement layer inhibits storage of carriers from the
first cladding layer into the low carrier concentration layer. Thus, the low carrier
concentration layer is kept in the depleted state and the electric capacitance between
the first cladding layer and the current blocking layer is kept small.
[0023] The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus,
the semiconductor laser device is more improved in high-frequency characteristic.
[0024] The thickness of the depletion enhancement layer is preferably at least 20 nm. Thus,
the semiconductor laser device is further improved in high-frequency characteristic.
[0025] The depletion enhancement layer, the low carrier concentration layer and the current
blocking layer may be successively formed on the first cladding layer except the current
injection region, and the semiconductor laser device may further comprise a second
cladding layer of a first conduction type provided to fill up a space enclosed with
the side surfaces of the depletion enhancement layer, the low carrier concentration
layer and the current blocking layer and the upper surface of the first cladding layer
in the current injection region.
[0026] In this case, the depletion enhancement layer inhibits storage of carriers from the
first cladding layer into the low carrier concentration layer. Thus, the low carrier
concentration layer is kept in the depleted state and the electric capacitance between
the first cladding layer and the current blocking layer is kept small.
[0027] The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus,
the semiconductor laser device is more improved in high-frequency characteristic.
[0028] The thickness of the depletion enhancement layer is preferably at least 20 nm. Thus,
the semiconductor laser device is further improved in high-frequency characteristic.
[0029] The depletion enhancement layer may have a single-layer structure or a superlattice
structure.
[0030] The active layer may include a layer made of (Al
x1Ga
1- x1)y1In
1-y1P, the depletion enhancement layer may be made of (Al
x2Ga
1-x2)
y2In
1-y2P or Al
x2Ga
1-x2As, the low carrier concentration layer may be made of (Al
x3Ga
1-x3)
y3In
1-y3P or Al
x3Ga
1-x3As, the current blocking layer may be made of (Al
x4Ga
1-x4)
y4In
1-y4P or Al
x4Ga
1-x4As, and x1, x2, x3, x4, y1, y2, y3 and y4 may be at least zero and not more than 1
respectively.
[0031] The active layer may include a layer made of Al
x1Ga
1-x1As, the depletion enhancement layer may be made of Al
x2Ga
1-x2As, the low carrier concentration layer may be made of Al
x3Ga
1-x3As, the current blocking layer may be made of Al
x4Ga
1-x4As , and x1, x2, x3 and x4 may be at least zero and not more than 1 respectively.
[0032] The active layer may be made of In
x1Ga
1-x1N, the depletion enhancement layer may be made of Al
x2Ga
1-x2N, the low carrier concentration layer may be made of Al
x3Ga
1-x3N, the current blocking layer may be made of Al
x4Ga
1-x4N, and x1, x2, x3 and x4 may be at least zero and not more than 1 respectively.
[0033] The active layer preferably includes a layer made of (Al
x1Ga
1-x1)
y1In
1-y1P, the depletion enhancement layer is preferably made of (Al
x2Ga
1-x2)
y2In
1-y2P, the low carrier concentration layer is preferably made of Al
x3Ga
1-x3As, the current blocking layer is preferably made of Al
x4Ga
1-x4As, x1, x2, x3, x4, y1 and y2 are preferably at least zero and not more than 1 respectively,
and the first conduction type is preferably the p type, and the second conduction
type is preferably the n type.
[0034] In this case, improvement of the high-frequency characteristic resulting from the
depletion enhancement layer inhibiting storage of carriers from the first cladding
layer into the low carrier concentration layer is particularly remarkable.
[0035] A semiconductor laser device according to another aspect of the present invention
comprises an active layer, a first claddinglayer of a first conduction type provided
on the active layer, a first current blocking layer having a low carrier concentration
provided on the first cladding layer except a current injection region and a depletion
enhancement layer formed between the first cladding layer and the first current blocking
layer for inhibiting storage of carriers in the first current blocking layer, while
the depletion enhancement layer has an energy level in band gap supplying second conduction
type carriers to compensate for first conduction type carriers supplied from the first
cladding layer due to a modulation doping effect.
[0036] The first current blocking layer having a low carrier concentration is an undoped
layer or a layer doped with a low density of impurity in a range capable of blocking
a current.
[0037] In the semiconductor laser device, the depletion enhancement layer formed with the
energy level in band gap supplying the second conduction type carriers is formed between
the first cladding layer and the first current blocking layer.
[0038] In this case, the second conduction type carriers supplied from the energy level
in band gap of the depletion enhancement layer compensate for the first conduction
type carriers supplied from the first cladding layer. Therefore, storage of carriers
can be prevented in the first current blocking layer having a low carrier concentration.
Thus, the first current blocking layer is kept in a depleted state. Therefore, electric
capacitance generated between the first current blocking layer and the first cladding
layer can be reduced and the operating speed of the semiconductor laser device can
be sufficiently increased.
[0039] At this point, the first current blocking layer has a narrower band gap than the
first cladding layer. When the first current blocking layer has a narrower band gap
than the first cladding layer, carriers are readily injected from the first cladding
layer into the first current blocking layer and stored therein. In this case, however,
the depletion enhancement layer formed between the first cladding layer and the first
current blocking layer can inhibit storage of carriers in the first current blocking
layer.
[0040] The energy level in band gap preferably has such density that substantially all energy
level in band gap ionize under a condition applying no bias voltage voltage. In this
case, it is possible to effectively compensate for the first conduction type carriers
supplied from the first cladding layer. Therefore, storage of carriers in the first
current blocking layer having a low carrier concentration can be more effectively
inhibited.
[0041] The energy level in band gap may be formed by doping with a second conduction type
impurity. In this case, the depletion enhancement layer provided with the energy level
in band gap can be readily formed.
[0042] The material of the depletion enhancement layer may be the same as the material of
the first current blocking layer. In this case, the band gap width of the depletion
enhancement layer and the first current blocking layer are equalized with each other.
[0043] The first cladding layer may have a larger band gap than the depletion enhancement
layer, and the semiconductor laser device may further comprise an intermediate band
gap layer provided between the first cladding layer and the depletion enhancement
layer and having a band gap smaller than the band gap of the first cladding layer
and larger than the band gap of the depletion enhancement layer.
[0044] In this case, carriers are hardly injected from the first cladding layer into the
depletion enhancement layer and hardly injected into the first current blocking layer
having a low carrier concentration either due to the intermediate band gap layer provided
between the first cladding layer and the depletion enhancement layer. In this case,
further, the carriers are injected into both of the depletion enhancement layer and
the intermediate band gap layer in a divided manner, and hence hardly injected into
the first current blocking layer.
[0045] Thus, storage of carriers in the first current blocking layer is further inhibited.
[0046] Further, the ranges of the thickness and the carrier concentration of the depletion
enhancement layer capable increasing the operating speed of the semiconductor layer
device are widened by providing the intermediate band gap layer in the aforementioned
manner. Therefore, the thickness and the carrier concentration of the depletion enhancement
layer can be readily set so that the depletion enhancement layer can be readily prepared.
[0047] The depletion enhancement layer may have a band gap smaller than the band gap of
the first cladding layer and larger than the band gap of the first current blocking
layer. In this case, the depletion enhancement layer serves as the aforementioned
intermediate band gap layer, thereby further inhibiting storage of carriers in the
first current blocking layer.
[0048] Also in this case, the ranges of the thickness and the carrier concentration of the
depletion enhancement layer capable of increasing the operating speed of the semiconductor
laser device are widened. Thus, the thickness and the carrier concentration of the
depletion enhancement layer can be readily set so that the depletion enhancement layer
can be readily prepared.
[0049] The first cladding layer may have a flat portion formed on the active layer and a
ridge portion formed on the flat portion in the current injection region, the depletion
enhancement layer may be formed on the flat portion located on both sides of the ridge
portion and on the side surfaces of the ridge portion, and the first current blocking
layer may be formed on the depletion enhancement layer. In this case, a ridge guided
semiconductor laser device improved in-operating speed is implemented.
[0050] The depletion enhancement layer and the first current blocking layer may be successively
formed on the first cladding layer except the current injection region, and the semiconductor
laser device may further comprise a second cladding layer of a first conduction type
provided to fill up a space enclosed with the side surfaces of the depletion enhancement
layer and the first current blocking layer and the upper surface of the first cladding
layer in the current injection region. In this case, a self-aligned semiconductor
laser device improved in operating speed is implemented.
[0051] The depletion enhancement layer may be formed on a region excluding the current injection
region. In this case, a current is quickly injected into the current injection region
provided with no depletion enhancement layer of the opposite conduction type.
[0052] The semiconductor laser device may further comprise a second current blocking layer
of a second conduction type provided on the first current blocking layer.
[0053] A semiconductor laser device according to another aspect of the present invention
comprises an active layer, a first cladding layer of a first conduction type provided
on the active layer, a first current blocking layer having a low carrier concentration
provided on the first cladding layer except a current injection region and a depletion
enhancement layer formed between the first cladding layer and the first current blocking
layer for inhibiting storage of carriers in the first current blocking layer.
[0054] In the semiconductor laser device, the depletion enhancement layer inhibits storage
of carriers from the first cladding layer into the first current blocking layer having
a low carrier concentration. Thus, the first current blocking layer having a low carrier
concentration is kept in a depleted state. Therefore, electric capacitance between
the first current blocking layer having a low carrier concentration and the first
cladding layer is kept small for sufficiently increasing the operating speed of the
semiconductor laser device.
[0055] At this point, the first current blocking layer having a low concentration has a
narrower band gap than the first cladding layer. When the first current blocking layer
having a low carrier concentration has a narrower band gap than the first cladding
layer, carriers are readily injected from the first cladding layer into the first
current blocking layer having a low carrier concentration and stored therein. In this
case, however, the depletion enhancement layer formed between the first cladding layer
and the first current blocking layer having a low carrier concentration can inhibit
storage of carriers in the first current blocking layer having a low carrier concentration.
[0056] The band gaps of the first cladding layer, the depletion enhancement layer and the
first current blocking layer having a low carrier concentration may be reduced in
this order.
[0057] Thus, the depletion enhancement layer having an intermediate band gap is provided
between the first cladding layer having a large band gap and the first current blocking
layer having a low carrier concentration having a small band gap.
[0058] In this case, the band offset between the first cladding layer and the depletion
enhancement layer is smaller than the band offset between the first cladding layer
and the first current blocking layer having a low carrier concentration, whereby carriers
are hardly injected from the first cladding layer into the depletion enhancement layer
while carriers are more hardly injected into the first current blocking layer having
a low carrier concentration. Further, the carriers are injected from the first cladding
layer into both of the first current blocking layer having a low carrier concentration
and the depletion enhancement layer in a divided manner, whereby the quantity of carriers
stored in the low carrier concentration layer is reduced. Thus, storage of carriers
in the first current blocking layer having a low carrier concentration can be inhibited
by the simple structure of setting the band gap of the depletion enhancement layer
to the intermediate level between the first current blocking layer having a low carrier
concentration and the first cladding layer.
[0059] The first cladding layer may have a flat portion formed on the active layer and a
ridge portion formed on the flat portion in the current injection region, the depletion
enhancement layer may be formed on the flat portion located on both sides of the ridge
portion and on the side surfaces of the ridge portion, and the first current blocking
layer having a low carrier concentration may be formed on the depletion enhancement
layer.
[0060] In this case, the depletion enhancement layer inhibits storage of carriers from the
flat portion of the first cladding layer into the first current blocking layer having
a low carrier concentration. Thus, the first current blocking layer having a low carrier
concentration is kept in the depleted state, and the electric capacitance between
the flat portion of the first cladding layer and the first current blocking layer
having a low carrier concentration is kept small.
[0061] The semiconductor laser device may further comprise a ridge-shaped second cladding
layer of a first conduction type provided on the depletion enhancement layer in the
current injection region, the depletion enhancement layer may be formed on the first
cladding layer, and the first current blocking layer having a lower carrier concentration
may be formed on the depletion enhancement layer located on both sides of the second
cladding layer and on the side surfaces of the second cladding layer.
[0062] In this case, the depletion enhancement layer inhibits storage of carriers from the
first cladding layer into the first current blocking layer having a low carrier concentration
layer. Thus, the first current blocking layer having a low carrier concentration is
kept in the depleted state and the electric capacitance between the first cladding
layer and the first current blocking layer having a low carrier concentration is kept
small.
[0063] The depletion enhancement layer and the first current blocking layer having a low
carrier concentration may be successively formed on the first cladding layer except
the current injection region, and the semiconductor laser device may further comprise
a second cladding layer of a first conduction type provided to fill up a space enclosed
with the side surfaces of the depletion enhancement layer and the first current blocking
layer having a low carrier concentration layer and the upper surface of the first
cladding layer in the current injection region.
[0064] In this case, the depletion enhancement layer inhibits storage of carriers from the
first cladding layer into the first current blocking layer having a low carrier concentration.
Thus, the first current blocking layer having a low carrier concentration is kept
in the depleted state and the electric capacitance between the first cladding layer
and the first current blocking layer having a low concentration is kept small.
[0065] The foregoing and other objects, features, aspects and advantages of the present
invention will become more apparent from the following detailed description of the
present invention when taken in conjunction with the accompanying drawings.
IN THE DRAWINGS:
[0066]
Fig. 1 is a typical sectional view of a semiconductor laser device according to a
first embodiment of the present invention;
Fig. 2 is an energy band diagram of valence bands of a p-type cladding layer, a depletion
enhancement layer and a low carrier concentration layer in the semiconductor laser
device shown in Fig. 1;
Figs. 3 to 5 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 1;
Fig. 6 is a diagram showing results of measurement of the relation between a cutoff
frequency and -the thickness of the depletion enhancement layer in the semiconductor
laser device shown in Fig. 1;
Fig. 7 illustrates an effect of improving the cutoff frequency by doping the depletion
enhancement layer of the semiconductor laser device according to the first embodiment;
Fig. 8 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in a semiconductor laser
device according to a second embodiment of the present invention;
Fig. 9 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in a semiconductor laser
device according to a third embodiment of the present invention;
Fig. 10 is a typical sectional view of a semiconductor laser device according to a
fourth embodiment of the present invention;
Figs. 11 to 13 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 10;
Fig. 14 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in the semiconductor
laser device according to the fourth embodiment;
Fig. 15 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in a semiconductor laser
device according to a fifth embodiment of the present invention;
Fig. 16 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in a semiconductor laser
device according to a sixth embodiment of the present invention;
Fig. 17 is a typical sectional view of a semiconductor laser device according to a
seventh embodiment of the present invention;
Figs. 18 and 19 are typical sectional views showing steps of a method of fabricating
the semiconductor laser device shown in Fig. 17;
Fig. 20 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in the semiconductor
laser device according to the seventh embodiment;
Fig. 21 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in a semiconductor laser
device according to an eighth embodiment of the present invention;
Fig. 22 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in a semiconductor laser
device according to a ninth embodiment of the present invention;
Fig. 23 is a typical sectional view of a semiconductor laser device according to each
of tenth to twelfth embodiments of the present invention;
Figs. 24(a) and 24(b) are energy band diagrams of a p-type cladding layer, a n-type
depletion enhancement layer and a first current blocking layer having a low carrier
concentration in the semiconductor laser device shown in Fig. 23;
Figs. 25 to 27 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 23;
Fig. 28 is a typical sectional view of a semiconductor laser device according to a
thirteenth embodiment of the present invention;
Fig. 29 is a typical sectional view of a semiconductor laser device according to a
fourteenth embodiment of the present invention;
Figs. 30 and 31 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 29;
Fig. 32 is a diagram showing results of measurement of the relation between a cutoff
frequency and the thickness of a depletion enhancement layer in the semiconductor
laser device according to the fourteenth embodiment;
Figs. 33(a) and 33(b) are diagrams for illustrating the principle and the function
of the present invention;
Fig. 34 is a typical sectional view showing the structure of a conventional semiconductor
laser device; and
Fig. 35 is an energy band diagram of valence bands of a p-type cladding layer and
a current blocking layer having a low carrier concentration in the semiconductor laser
device shown in Fig. 34.
(1) First Embodiment
[0067] Fig. 1 is a typical sectional view of a semiconductor laser device according to a
first embodiment of the present invention.
[0068] In the semiconductor laser device shown in Fig. 1, a cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 1500 nm and an emission layer 14 described later are successively
formed on an n-GaAs substrate 1. A cladding layer 2 of p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 1500 nm and a contact layer of p-Ga
0.5In
0.5P having a thickness of 200 nm are successively formed on the emission layer 14. The
p-type cladding layer 6 and the p-type contact layer 7 are etched to define a ridge
portion.
[0069] The carrier concentration of the n-GaAs substrate 1 is 1 × 10
18 cm
-3, the carrier concentration of the n-type cladding layer 2 is 3 x 10
17 cm
-3, and the carrier concentration of the p-type contact layer 7 is 2 × 10
18 cm
-3 respectively.
[0070] Further, a depletion enhancement layer 8 of a thickness t having a striped opening
on the upper surface of the ridge portion is formed on the p-type cladding layer 6.
A low carrier concentration layer 9 of GaAs of 1000 nm in thickness having a striped
opening on the upper surface of the ridge portion is formed on the depletion enhancement
layer 8. An n-type current blocking layer 10 of n-GaAs of 500 nm in thickness having
a striped opening on the upper surface of the ridge portion is formed on the low carrier
concentration layer 9. The carrier concentration of the n-type current blocking layer
10 is 8 x 10
17 cm
-3. The carrier concentration of the low carrier concentration layer 9 is lower than
that of the n-type current blocking layer 10.
[0071] A contact layer 11 of p-GaAs having a thickness of 3000 nm is formed on the p-type
contact layer 7 located in the striped opening of the n-type current blocking layer
10 and on the n-type current blocking layer 10. The carrier concentration of the p-type
contact layer is 3 × 10
19 cm
-3. A p-electrode 12 having a thickness of 300 nm is formed on the p-type contact layer
11. An n electrode 13 having a thickness of 300 nm is formed on the back side of the
n-GaAs substrate 1.
[0072] The emission layer 14 includes a guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm formed on the n-type cladding layer 2, a quantum well
active layer 4 formed on the guide layer 3 and a guide layer 5 of (Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm formed on the quantum well active layer 4.
[0073] The quantum well active layer 4 has a superlattice structure formed by alternately
stacking a plurality of quantum well layers 15 of Ga
0.5In
0.5P each having a thickness of 5 nm and a plurality of barrier layers 16 of (Al
0.5Ga
0.5)
0.5In
0.5P each having a thickness of 5 nm. For example, the number of the barrier layers 16
is 2,and the number of the quantumwell layers 15 is 3.
[0074] Table 1 shows the aforementioned structure.

[0075] Fig. 2 typically shows an energy band diagram of valence bands of the p-type cladding
layer 6, the depletion enhancement layer 8 and the low carrier concentration layer
9 of the semiconductor laser device shown in Fig. 1.
[0076] As shown in Fig. 2, the band gaps of the p-type cladding layer 6, the depletion enhancement
layer 8 and the low carrier concentration layer 9 are reduced in this order. Therefore,
the band offset between the p-type cladding layer 6 and the depletion enhancement
layer 8 in contact therewith is reduced as compared with the band offset between the
p-type cladding layer 6 and the low carrier concentration layer 9 so that carriers
are hardly injected from the p-type cladding layer 6 into the depletion enhancement
layer 8 and hardly injected into the low carrier concentration layer 9 either. Consequently,
the quantity of carriers stored in the low carrier concentration layer 9 is reduced.
Further, carriers are injected into both of the low carrier concentration layer 9
and the depletion enhancement layer 8 in a divided manner, whereby the quantity of
the carriers stored in the low carrier concentration layer 9 is reduced.
[0077] The low carrier concentration layer 9, storing a small quantity of carriers, is kept
in a depleted state, whereby the electric capacitance between the n-type current blocking
layer 10 and the p-type cladding layer 6 is kept small for sufficiently increasing
the operating speed of the semiconductor laser device.
[0078] Thus, the high-frequency characteristic of the semiconductor laser device shown in
Fig. 1 is improved through the simple structure of setting the band gap of the depletion
enhancement layer 8 to the intermediate level between those of the low carrier concentration
layer 9 and the p-type cladding layer 6.
[0079] Figs. 3, 4 and 5 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 1.
[0080] As shown in Fig. 3, the cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P, the guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P, the quantum well active layer 4, the guide layer 5 of (Al
0.5Ga
0.5)
0.5In
0.5P, the cladding layer 6 of (Al
0.7Ga
0.3)
0.5In
0.5P and the contact layer 7 of p-Ga
0.5In
0.5P are successively grown on the n-GaAs substrate 1 by MOCVD (metal-organic chemical
vapor deposition).
[0081] As shown in Fig. 4, an SiO
2 film is formed on the p-type contact layer 7 and patterned for forming a striped
SiO
2 film 17. Thereafter the p-type contact layer 7 and the p-type cladding layer 6 are
partially removed by etching through the SiO
2 film 17 serving as a mask, for forming the ridge portion.
[0082] As shown in Fig. 5, the depletion enhancement layer 8 of Ga
0.5In
0.5P, the low carrier concentration layer 9 of GaAs and the current blocking layer 10
of n-GaAs are successively grown on the p-type cladding layer 6 by MOCVD through the
SiO
2 film 17 serving as a selective growth mask.
[0083] The SiO
2 film 17 is removed and thereafter the contact layer 11 of p-GaAs is formed on the
n-type current blocking layer 10 and on the p-type contact layer 7 by MOCVD as shown
in Fig. 1, while the p-electrode 12 of Cr/Au is formed on the surface of the p-type
contact layer 11 and the n electrode 13 of AuGe/Ni/Au is formed on the back side of
the n-GaAs substrate 1.
[0084] Fig. 6 is a diagram showing the results of measurement of a cutoff frequency of the
semiconductor laser device shown in Table 1 with variation of the thickness t of the
depletion enhancement layer 8. The cutoff frequency is such a frequency that the amplitude
of a laser beam superposed with a sine wave output from the object semiconductor laser
device is reduced by 3 dB as compared with the case of superposing a low frequency
(the superposed frequency is not more than 10 MHz in this example). Referring to Fig.
6, ○ denotes a case of employing a depletion enhancement layer 8 of Ga
0.5In
0.5P having a single-layer structure, □ denotes a case of employing a depletion enhancement
layer 8 of a superlattice structure alternately having (Al
0.7Ga
0.3)
0.5In
0.5P barrier layers and Ga
0.5In
0.5P well layers (the thickness t is the sum of the thicknesses of the well layers),
and △ denotes a case of employing a depletion enhancement layer 8 of Al
0.45Ga
0.55As having a single-layer structure respectively.
[0085] The cutoff frequency, 200 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 8, is improved when the thickness t of the depletion
enhancement layer 8 is increased, remarkably improved when the thickness t of the
depletion enhancement layer 8 exceeds 10 nm, and substantially saturated when the
thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 8 is preferably at least 10 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 8 is at least 15 nm, the intermediate level between 10 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
[0086] Fig. 7 illustrates an effect of improving the cutoff frequency by doping the depletion
enhancement layer 8 of the semiconductor laser device shown in Fig. 1. The horizontal
axis shows the ratio of the carrier concentration of the depletion enhancement layer
8 to the carrier concentration of the p-type cladding layer 6, and the vertical axis
shows the cutoff frequency. The depletion enhancement layer 8 of this semiconductor
laser device is made of p-type GaInP, and the thickness t thereof is 25 nm.
[0087] As shown in Fig. 7, the effect of improving the cutoff frequency is small when the
carrier concentration of the depletion enhancement layer 8 is higher than that of
the p-type cladding layer 6, while a large effect of improving the cutoff frequency
is attained when the carrier concentration of the depletion enhancement layer 8 is
lower than that of the p-type cladding layer 6. Therefore, the carrier concentration
of the depletion enhancement layer 8 is preferably lower than that of the p-type cladding
layer 6.
(2) Second Embodiment
[0088] A semiconductor laser device according to a second embodiment of the present invention
is now described.
[0089] The structure of the semiconductor laser device according to the second embodiment
is similar to that shown in Fig. 1, while the materials, thicknesses and carrier concentrations
of respective layers are different from those in the first embodiment. Table 2 shows
the materials, thicknesses and carrier concentrations of the respective layers forming
the semiconductor laser device according to this embodiment.

[0090] Fig. 8 illustrates results of measurement of a cutoff frequency of the semiconductor
laser device shown in Table 2 with variation of the thickness t of a depletion enhancement
layer 8. Referring to Fig. 8, ○ denotes a case of employing a depletion enhancement
layer 8 of Al
0.25Ga
0.75As having a single-layer structure, and □ denotes a case of employing a depletion
enhancement layer 8 of a superlattice structure alternately having Al
0.45Ga
0.55As barrier layers and Al
0.25Ga
0.75As well layers (the thickness t is the sum of the thicknesses of the well layers).
[0091] The cutoff frequency, 400 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 8, is improved when the thickness t of the depletion
enhancement layer 8 is increased, remarkably improved when the thickness t of the
depletion enhancement layer 8 exceeds 10 nm, and substantially saturated when the
thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 8 is preferably at least 10 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 8 is at least 15 nm, the intermediate level between 10 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(3) Third Embodiment
[0092] A semiconductor laser device according to a third embodiment of the present invention
is now described.
[0093] The structure of the semiconductor laser device according to the third embodiment
is similar to that shown in Fig. 1, while the materials, thicknesses and carrier concentrations
of respective layers are different from those in the first embodiment. Table 3 shows
the materials, thicknesses and carrier concentrations of the respective layers forming
the semiconductor laser device according to this embodiment.

[0094] Fig. 9 illustrates results of measurement of a cutoff frequency of the semiconductor
laser device shown in Table 3 with variation of the thickness t of a depletion enhancement
layer 8. Referring to Fig. 9, ○ denotes a case of employing a depletion enhancement
layer 8 of Al
0.07Ga
0.93N having a single-layer structure, and □ denotes a case of employing a depletion enhancement
layer 8 of a superlattice structure alternately having Al
0.15Ga
0.85N barrier layers and Al
0.07Ga
0.93N well layers (the thickness t is the sum of the thicknesses of the well layers).
[0095] The cutoff frequency, 320 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 8, is gradually improved when the thickness t of the
depletion enhancement layer 8 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 8 exceeds 10 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 8 is preferably at least 10 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 8 is at least 15 nm, the intermediate level between 10 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(4) Fourth Embodiment
[0096] Fig. 10 is a typical sectional view showing a semiconductor laser device according
to a fourth embodiment of the present invention.
[0097] In the semiconductor laser device shown in Fig. 10, respective layers 2 to 5 are
formed on an n-GaAs substrate 1, similarly to the semiconductor laser device shown
in Fig. 1.
[0098] A cladding layer 61 of p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 200 nm and a depletion enhancement layer 62 of Ga
0.5In
0.5P are successively formed on the guide layer 5. The carrier concentration of the p-type
cladding layer 61 is 3 × 10
17 cm
-3.
[0099] A cladding layer 63 of p-(Al
0.7Ga
0.3)
0.5In
0.5P of 1300 nm in thickness having a ridge shape is formed on the depletion enhancement
layer 62. The carrier concentration of the p-type cladding layer 63 is 3 × 10
17 cm
-3. A contact layer 7 of p-Ga
0.5In
0.5P is formed on the upper surface of the p-type cladding layer 63.
[0100] A low carrier concentration layer 9 of GaAs of 1000 nm in thickness having a striped
opening on the upper surface of the p-type contact layer 7 is formed on portions of
the depletion enhancement layer 62 located on both sides of the p-type cladding layer
63 and on the side surfaces of the p-type cladding layer 63.
[0101] Further, a current blocking layer 10 of n-GaAs of 500 nm in thickness having a striped
opening on the upper surface of the ridge portion is formed on the low carrier concentration
layer 9. A p-type contact layer 11 is formed on the p-type contact layer 7 and the
n-type current blocking layer 10.
[0102] Table 4 shows the aforementioned structure.

[0103] Figs. 11, 12 and 13 are typical sectional views showing steps in a method of fabricating
the semiconductor layer device shown in Fig. 10.
[0104] As shown in Fig. 11, the cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P , the guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P, the quantum well active layer 4, the guide layer 5 of (Al
0.5Ga
0.5)
0.5In
0.5P, the cladding layer 61 of (Al
0.7Ga
0.3)
0.5In
0.5P, the depletion enhancement layer 62 of Ga
0.5In
0.5P, the cladding layer 63 of p-(Al
0.7Ga
0.3)
0.5In
0.5P and the contact layer 7 of p-Ga
0.5In
0.5P are successively grown on the n-GaAs substrate 1 by MOCVD.
[0105] As shown in Fig. 12, an SiO
2 film is formed on the p-type contact layer 7 and patterned for forming a striped
SiO
2 film 64. Thereafter the p-type contact layer 7 and the p-type cladding layer 63 are
removed by etching through the SiO
2 film 64 serving as a mask, for forming the ridge portion.
[0106] As shown in Fig. 13, the low carrier concentration layer 9 of GaAs and the current
blocking layer 10 of n-GaAs are successively grown on the side surfaces of the depletion
enhancement layer 62 and the p-type cladding layer 63 located on both sides of the
ridge portion by MOCVD through the SiO
2 film 64 serving as a selective growth mask.
[0107] The SiO
2 film 64 is removed and thereafter the contact layer 11 of p-GaAs is formed on the
n-type current blocking layer 10 and the p-type contact layer 7 by MOCVD, while forming
a p-electrode 12 of Cr/Au on the surface of the p-type contact layer 11 and forming
an n electrode 13 of AuGe/Ni/Au on the back side of the n-GaAs substrate 1, as shown
in Fig. 10.
[0108] Fig. 14 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 4 with variation of the thickness t
of the depletion enhancement layer 62. Referring to Fig. 14, ○ denotes a case of employing
a depletion enhancement layer 62 of Ga
0.5In
0.5P having a single-layer structure, denotes a case of employing a depletion enhancement
layer 62 of a superlattice structure alternately having (Al
0.7Ga
0.3)
0.5In
0.5P barrier layers and Ga
0.5In
0.5P well layers (the thickness t is the sum of the thicknesses of the well layers),
and denotes a case of employing a depletion enhancement layer 62 of Al
0.45Ga
0.55As having a single-layer structure respectively.
[0109] The cutoff frequency, 200 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 62, is gradually improved when the thickness t of the
depletion enhancement layer 62 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 62 exceeds 15 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 62 is preferably at least 15 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 62 is at least 18 nm, the intermediate level between 15 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(5) Fifth Embodiment
[0110] A semiconductor laser device according to a fifth embodiment of the present invention
is now described.
[0111] The structure of the semiconductor laser device according to the fifth embodiment
is similar to that shown in Fig. 10, except the material, thicknesses and carrier
concentrations of respective layers. Table 5 shows the materials, thicknesses and
carrier concentrations of the respective layers of the semiconductor laser device
according to this embodiment.

[0112] Fig. 15 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 5 with variation of the thickness t
of a depletion enhancement layer 62. Referring to Fig. 15, ○ denotes a case of employing
a depletion enhancement layer 62 of Al
0.25Ga
0.75As having a single-layer structure, and □ denotes a case of employing a depletion
enhancement layer 62 of a superlattice structure alternately having Al
0.45Ga
0.55As barrier layers and Al
0.25Ga
0.75As well layers (the thickness t is the sum of the thicknesses of the well layers)
respectively.
[0113] The cutoff frequency, 400 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 62, is gradually improved when the thickness t of the
depletion enhancement layer 62 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 62 exceeds 15 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 62 is preferably at least 15 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 62 is at least 18 nm, the intermediate level between 15 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(6) Sixth Embodiment
[0114] A semiconductor laser device according to a sixth embodiment of the present invention
is now described.
[0115] The structure of the semiconductor laser device according to the sixth embodiment
is similar to that shown in Fig. 10, except the materials, thicknesses and carrier
concentrations of respective layers. Table 6 shows the materials, thicknesses and
carrier concentrations of the respective layers of the semiconductor laser device
according to this embodiment.

[0116] Fig. 16 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 6 with variation of the thickness t
of a depletion enhancement layer 62. Referring to Fig. 15, ○ denotes a case of employing
a depletion enhancement layer 62 of Al
0.07Ga
0.93N having a single-layer structure, and □ denotes a case of employing a depletion enhancement
layer 62 of a superlattice structure alternately having Al
0.15Ga
0.85N barrier layers and Al
0.07Ga
0.93N well layers (the thickness t is the sum of the thicknesses of the well layers) respectively.
[0117] The cutoff frequency, 320 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 62, is gradually improved when the thickness t of the
depletion enhancement layer 62 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 62 exceeds 15 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 62 is preferably at least 15 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 62 is at least 18 nm, the intermediate level between 15 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(7) Seventh Embodiment
[0118] Fig. 17 is a typical sectional view of a semiconductor laser device according to
a seventh embodiment of the present invention.
[0119] In the semiconductor laser device shown in Fig. 17, respective layers 2 to 5 are
formed on an n-GaAs substrate 1, similarly to the semiconductor laser device shown
in Fig. 1.
[0120] A cladding layer 91 of p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 200 nm is formed on the guide layer 5. The carrier concentration
of the p-type cladding layer 91 is 3 x 10
17 cm
-3.
[0121] A depletion enhancement layer 92 of Ga
0.5In
0.5P, a low carrier concentration 93 of GaAs having a thickness of 1000 nm and a current
blocking layer 94 of n-GaAs having a thickness of 500 nm are successively formed on
the p-type cladding layer 91.
[0122] Central regions of the depletion enhancement layer 92, the low carrier concentration
layer 93 and the n-type current blocking layer 94 are removed to define a striped
opening. The carrier concentration of the n-type current blocking layer 94 is 8 ×
10
17 cm
-3. The carrier concentration of the low carrier concentration layer 93 is lower than
that of the n-type current blocking layer 94.
[0123] A p-type cladding layer 95 p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 1300 nm is formed on the p-type cladding layer 91 and the
n-type current blocking layer 94 to fill up the striped opening. The carrier concentration
of the p-type cladding layer 95 is 3 × 10
17 cm
-3.
[0124] A contact layer 96 of p-Ga
0.5In
0.5P having a thickness of 200 nm is formed on the p-type cladding layer 95. A contact
layer 97 of p-GaAs having a thickness of 3000 nm is formed on the p-type contact layer
96. The carrier concentration of the p-type contact layer 96 is 2 × 10
18 cm
-3. The carrier concentration of the p-type contact layer 97 is 3 × 10
19 cm
-3.
[0125] Table 7 shows the aforementioned structure.

[0126] Figs. 18 and 19 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 17.
[0127] As shown in Fig. 18, the cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P, the guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm, the quantum well active layer 4, the guide layer of
(Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm, the cladding layer 91 of p-(Al
0.7Ga
0.3)
0.5 In
0.5P, the depletion enhancement layer 92 of Ga
0.5In
0.5P, the low carrier concentration layer 93 of GaAs and the current blocking layer 94
of n-GaAs are successively grown on the n-GaAs substrate 1 by MOCVD.
[0128] A mask (not shown) is formed on the n-type current blocking layer 94 and patterned
to have a striped opening. Thereafter the central portions of the n-type current blocking
layer 94, the low carrier concentration layer 93 and the depletion enhancement layer
92 are removed by etching for forming the striped opening, as shown in Fig. 19.
[0129] Then, the cladding layer 95 of p-(Al
0.7Ga
0.3)
0.5In
0.5P, the contact layer 96 of p-Ga
0.5In
0.5P and the contact layer 97 of p-GaAs are successively formed on the n-type current
blocking layer 94 and on the p-type cladding layer 91 located in the striped opening
by MOCVD, as shown in Fig. 17. A p-electrode 12 of Cr/Au is formed on the surface
of the p-type contact layer 97, and an n electrode 13 of AuGe/Ni/Au is formed on the
back side of the n-GaAs substrate 1.
[0130] Fig. 20 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 7 with variation of the thickness t
of the depletion enhancement layer 92. Referring to Fig. 20, ○ denotes a case of employing
a depletion enhancement layer 92 of Ga
0.5In
0.5P having a single-layer structure, □ denotes a case of employing a depletion enhancement
layer 92 of a superlattice structure alternately having (Al
0.7Ga
0.3)
0.5In
0.5P barrier layers and Ga
0.5In
0.5P well layers (the thickness t is the sum of the thicknesses of the well layers),
and △ denotes a case of employing a depletion enhancement layer 92 of Al
0.45Ga
0.55As having a single-layer structure respectively.
[0131] The cutoff frequency, 200 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 92, is gradually improved when the thickness t of the
depletion enhancement layer 92 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 92 exceeds 10 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 92 is preferably at least 15 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 92 is at least 18 nm, the intermediate level between 15 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(8) Eighth Embodiment
[0132] A semiconductor laser device according to an eighth embodiment of the present invention
is now described.
[0133] The structure of the semiconductor laser device according to the eighth embodiment
is similar to that shown in Fig. 17, except the materials, thicknesses and carrier
concentrations of respective layers. Table 8 shows the materials, thicknesses and
carrier concentrations of the respective layers in the semiconductor laser device
according to this embodiment.

[0134] Fig. 21 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 8 with variation of the thickness t
of a depletion enhancement layer 92. Referring to Fig. 21, ○ denotes a case of employing
a depletion enhancement layer 92 of Al
0.25Ga
0.75As having a single-layer structure, and □ denotes a case of employing a depletion
enhancement layer 92 of a superlattice structure alternately having Al
0.45Ga
0.75As barrier layers and Al
0.25Ga
0.75As well layers (the thickness t is the sum of the thicknesses of the well layers)
respectively.
[0135] The cutoff frequency, 400 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 92, is gradually improved when the thickness t of the
depletion enhancement layer 92 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 92 exceeds 15 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 92 is preferably at least 15 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 92 is at least 18 nm, the intermediate level between 15 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
(9) Ninth Embodiment
[0136] A semiconductor laser device according to a ninth embodiment of the present invention
is now described.
[0137] The structure of the semiconductor laser device according to the ninth embodiment
is similar to that shown in Fig. 17, except the materials, thicknesses and carrier
concentrations of respective layers. Table 9 shows the materials, thicknesses and
carrier concentrations of the respective layers in the semiconductor laser device
according to this embodiment.

[0138] Fig. 22 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 9 with variation of the thickness t
of a depletion enhancement layer 92. Referring to Fig. 22, ○ denotes a case of employing
a depletion enhancement layer 92 of Al
0.07Ga
0.93N having a single-layer structure, and □ denotes a case of employing a depletion enhancement
layer 92 of a superlattice structure alternately having Al
0.15Ga
0.85N barrier layers and Al
0.07Ga
0.93N well layers (the thickness t is the sum of the thicknesses of the well layers) respectively.
[0139] The cutoff frequency, 320 MHz when the semiconductor laser device is formed with
no depletion enhancement layer 92, is gradually improved when the thickness t of the
depletion enhancement layer 92 is increased, remarkably improved when the thickness
t of the depletion enhancement layer 92 exceeds 15 nm, and substantially saturated
when the thickness t is about 20 nm. Therefore, the thickness t of the depletion enhancement
layer 92 is preferably at least 15 nm, and more preferably at least 20 nm saturating
improvement of the cutoff frequency. When the thickness t of the depletion enhancement
layer 92 is at least 18 nm, the intermediate level between 15 nm and 20 nm, the high-frequency
characteristic can be sufficiently improved.
[0140] The materials for the active layer, the depletion enhancement layer, the low carrier
concentration layer and the current blocking layer are not restricted to those in
the aforementioned embodiments. For example, any arbitrary combination of an active
layer of (Al
x1Ga
1-x1)
y1In
1-y1P, a depletion enhancement layer of (Al
x2Ga
1-x2)
y2In
1-y2P or Al
x2Ga
1-x2As, a low carrier concentration layer of (Al
x3Ga
1-x3)
y3In
1-y3P or Al
x3Ga
1-x3As and a current blocking layer of (Al
x4Ga
1-x4)
y4In
1-y4P or Al
x4Ga
1- x4AS can be employed. Each of x1, x2, x3, x4, y1, y2, y3 and y4 is at least zero and
not more than 1.
(10) Tenth Embodiment
[0141] Fig. 23 is a typical sectional view of a semiconductor laser device according to
a tenth embodiment of the present invention.
[0142] In the semiconductor laser device shown in Fig. 23, a cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 1500 nm and an emission layer 14 described later are successively
formed on an n-GaAs substrate 1. A cladding layer 6 of p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 1500 nm and a contact layer 7 of p-Ga
0.5In
0.5P having a thickness of 200 nm are successively formed on the emission layer 14. The
p-type cladding layer 6 and the p-type contact layer 7 are etched for defining a ridge
portion.
[0143] The carrier concentration of the n-GaAs substrate 1 is 1 × 10
18 cm
-3, the carrier concentration of the n-type cladding layer 2 is 3 × 10
17 cm
-3, the carrier concentration of the p-type cladding layer 6 is 3 x 10
17 cm
-3, and the carrier concentration of the p-type contact layer 7 is 2 x 10
18 cm
-3.
[0144] Further, a n-type depletion enhancement layer 8n of n-GaAs having a thickness t,
having a striped opening on the upper surface of the ridge portion, is formed on the
p-type cladding layer 6. A first current blocking layer 9 of undoped GaAs of 1000
nm in thickness having a low carrier concentration, having a striped opening on the
upper surface of the ridge portion, is formed on the n-type depletion enhancement
layer 8. A reverse conduction type second current blocking layer 10 of n-GaAs of 500
nm in thickness having a striped opening on the upper surface of the ridge portion
is formed on the first current blocking layer 9 having a low carrier concentration.
The carrier concentration of the n-type depletion enhancement layer 8n is 5 × 10
17 cm
-3. The carrier concentration of the second current blocking layer 10 is 8 × 10
17 cm
-3.
[0145] A contact layer 11 of p-GaAs having a thickness of 3000 nm is formed on the p-type
contact layer 7 located in the striped opening of the second current blocking layer
10 and on the second current blocking layer 10. The carrier concentration of the p-type
contact layer 11 is 3 x 10
19 cm
-3. A p-electrode 12 having a thickness of 300 nm is formed on the p-type contact layer
11. An n electrode 13 having a thickness of 300 nm is formed on the back side of the
n-GaAs substrate 1. Thus, the semiconductor laser device shown in Fig. 23 has a ridge
guided structure.
[0146] The emission layer 14 includes a guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm formed on the n-type cladding layer 2, a quantum well
active layer 4 formed on the guide layer 3 and a guide layer 5 of (Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm formed on the quantum well layer 4.
[0147] The quantum well active layer 4 has a superlattice structure formed by alternately
stacking a plurality of quantum well layers 15 of Ga
0.5In
0.5P each having a thickness of 5 nm and a plurality of barrier layers 16 of (Al
0.5Ga
0.5)
0.5In
0.5P each having a thickness of 5 nm. For example, the number of the barrier layers 16
is 2,and the number of the quantumwell layers 15 is 3.
[0148] Table 10 shows the aforementioned structure.

[0149] Figs. 24(a) and 24 (b) typically show energy band diagrams of the p-type cladding
layer 6, the n-type depletion enhancement layer 8 and the first current blocking layer
9 having a low carrier concentration in the semiconductor laser device shown in Fig.
23.
[0150] As shown in Fig. 24(a), the n-type depletion enhancement layer 8n formed with impurity
levels of the opposite conduction type to the p-type cladding layer 6, i.e., donor
levels, is formed between the p-type cladding layer 6 and the first current blocking
layer 9 having a low carrier concentration.
[0151] In this case, the band gap of the n-type depletion enhancement layer 8n and that
of the first current blocking layer 9 having a low carrier concentration are equal
to each other, and the band gaps of these layers 8n and 9 are smaller than that of
the p-type cladding layer 6.
[0152] As shown in Fig. 24(b), carriers supplied from the donor levels formed on the n-type
depletion enhancement layer 8n compensate for carriers supplied from the p-type cladding
layer 6. Therefore, the quantity of carriers stored in the first current blocking
layer 9 having a low carrier concentration is reduced.
[0153] The first current blocking layer 9 having a low carrier concentration is kept in
a depleted state due to such reduction of the quantity of carriers stored therein,
whereby electric capacitance generated between the first current blocking layer 9
having a low carrier concentration and the p-type cladding layer 6 can be reduced
for sufficiently increasing the operating speed of the semiconductor laser device.
Thus, the high-frequency characteristic of the semiconductor laser device shown in
Fig. 23 is readily improved by forming the n-type depletion enhancement layer 8 formed
with the impurity levels of the opposite conduction type to the p-type cladding layer
6 between the p-type cladding layer 6 and the first current blocking layer 9 having
a low carrier concentration.
[0154] Figs. 25, 26 and 27 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 23.
[0155] As shown in Fig. 25, the cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P, the guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P, the quantum well active layer 4, the guide layer 5 of (Al
0.5Ga
0.5)
0.5In
0.5P, the cladding layer 6 of p-(Al
0.7Ga
0.3)
0.5In
0.5P and the contact layer 7 of p-Ga
0.5In
0.5P are successively formed on the n-GaAs substrate 1 by MOCVD.
[0156] As shown in Fig. 26, an SiO
2 film is formed on the p-type contact layer 7 and patterned for forming a striped
SiO
2 film 17. Thereafter the p-type contact layer 7 and the p-type cladding layer 6 are
partially removed by etching through the SiO
2 film 17 serving as a mask, for defining the ridge portion.
[0157] As shown in Fig. 27, the depletion enhancement layer 8n of n-GaAs, the first current
blocking layer 9 of undoped GaAs having a low carrier concentration and the second
current blocking layer 10 of n-GaAs are successively formed on the p-type cladding
layer 6 by MOCVD through the SiO
2 film 17 serving as a selective growth mask.
[0158] The SiO
2 film 17 is removed and thereafter the contact layer 11 of p-GaAs is formed on the
second current blocking layer 10 and the p-type contact layer 7 by MOCVD, while the
p-electrode 12 of Cr/Au is formed on the surface of the p-type contact layer 11 and
the n electrode 13 of AuGe/Ni/Au is formed on the back side of the n-GaAs substrate
1 as shown in Fig. 23.
[0159] In the semiconductor laser device shown in Fig. 23, the thickness t of the depletion
enhancement layer 8n having a carrier concentration of 5 × 10
17 cm
-3 is varied for measuring the cutoff frequency of the semiconductor laser device at
each level of the thickness t. Consequently, it is proved that the cutoff frequency
is remarkably improved when the thickness t of the n-type depletion enhancement layer
8n is in the range of 20 to 35 nm.
[0160] Further, the thickness t of the n-type depletion enhancement layer 8n is set to 30
nm and the carrier concentration thereof is varied for measuring the cutoff frequency
of the semiconductor laser device at each level of the carrier concentration. Consequently,
it is provided that the cutoff frequency is remarkably improved when the carrier concentration
of the n-type depletion enhancement layer 8n is in the range of 3 x 10
17 to 6 x 10
17 cm
-3.
(11) Eleventh Embodiment
[0161] A semiconductor laser device according to an eleventh embodiment of the present invention
is now described.
[0162] The structure of the semiconductor laser device according to the eleventh embodiment
is similar to that shown in Fig. 23, except the materials, thicknesses and carrier
concentrations of respective layers. Table 11 shows the materials, thicknesses and
carrier concentrations of the respective layers of the semiconductor laser device
according to this embodiment.

[0163] As shown in Table 11, an n-type depletion enhancement layer 8n formed with impurity
levels (donor levels) of the opposite conduction type to a p-type cladding layer 6
is formed between the p-type cladding layer 6 and a first current blocking layer 9
having a low carrier concentration in the semiconductor laser device according to
this embodiment. In this embodiment, therefore, carriers supplied from the donor levels
formed on the n-type depletion enhancement layer 8n compensate for those supplied
from the p-type cladding layer 6, similarly to the tenth embodiment. Therefore, the
quantity of carriers stored in the first current blocking layer 9 having a low carrier
concentration is reduced.
[0164] The first current blocking layer 9 having a low carrier concentration is kept in
a depleted state due to such reduction of the quantity of carriers stored therein,
whereby electric capacitance generated between the first current blocking layer 9
having a low carrier concentration and the p-type cladding layer 6 can be reduced
for sufficiently increasing the operating speed of the semiconductor laser device.
Thus, the high-frequency characteristic of the semiconductor laser device is readily
improved in this embodiment.
[0165] According to this embodiment, the semiconductor laser device is so set that the band
gap of the n-type depletion enhancement layer 8n is smaller than that of the p-type
cladding layer 6 and larger than that of the first current blocking layer 9 having
a low carrier concentration. The n-type depletion enhancement layer 8n according to
this embodiment also has a function as an intermediate band gap layer described later
with reference to a thirteenth embodiment.
[0166] In this case, the band gaps of the p-type cladding layer 6, the n-type depletion
enhancement layer 8n and the first current blocking layer 9 having a low carrier concentration
are reduced in this order. Therefore, the band offset between the p-type cladding
layer 6 and the n-type depletion enhancement layer 8n is smaller than that between
the p-type cladding layer 6 and the first current blocking layer 9 having a low carrier
concentration. Thus, carriers are hardly injected from the p-type cladding layer 6
into the n-type depletion enhancement layer 8n, and hardly injected into the first
current blocking layer 9 having a low carrier concentration either. In this case,
further, carriers are injected into both of the n-type depletion enhancement layer
8n and the first current blocking layer 9 having a low carrier concentration in a
divided manner, whereby the quantity of carriers injected into the first current blocking
layer 9 having a low carrier concentration is reduced.
[0167] As hereinabove described, the quantity of carriers stored in the first current blocking
layer 9 having a low carrier concentration can be further reduced by forming the n-type
depletion enhancement layer 8n having the intermediate band gap between the p-type
cladding layer 6 and the first current blocking layer 9 having a low carrier concentration.
[0168] Thus, electric capacitance generated between the first current blocking layer 9 having
a low carrier concentration and the p-type cladding layer 6 can be further reduced
in the embodiment having the n-type depletion enhancement layer 8n, having the intermediate
band gap between the p-type cladding layer 6 and the first current blocking layer
9 having a low carrier concentration, whereby the operating speed of the semiconductor
laser device is further improved.
[0169] In the semiconductor laser device according to this embodiment, the thickness t of
the n-type depletion enhancement layer 8n having a carrier concentration of 5 × 10
17 cm
-3 is varied for measuring the cutoff frequency of the semiconductor laser device at
each level of the thickness t. Consequently, it is proved that the cutoff frequency
is remarkably improved when the thickness t of the n-type depletion enhancement layer
8n is in the range of 10 to 40 nm.
[0170] Further, the thickness t of the n-type depletion enhancement layer 8n is set to 25
nm and the carrier concentration thereof is varied for measuring the cutoff frequency
of the semiconductor laser device at each level of the carrier concentration. Consequently,
it is proved that the cutoff frequency is remarkably improved when the carrier concentration
of the n-type depletion enhancement layer 8n is in the range of 2.5 × 10
17 to 8.5 × 10
17 cm
-3.
[0171] As understood from the aforementioned results, the n-type depletion enhancement layer
8n has the intermediate band gap between the p-type cladding layer 6 and the first
current blocking layer 9 having a low carrier concentration in this embodiment, whereby
the ranges of the thickness and the carrier concentration of the n-type depletion
enhancement layer 8n capable of remarkably improving the cutoff frequency of the semiconductor
laser device are widened as compared with the first embodiment provided with the n-type
depletion enhancement layer 8 and the first current blocking layer 9 of a low carrier
concentration having the same band gaps. Thus, the thickness and the carrier concentration
of the n-type depletion enhancement layer 8n can be readily set and the n-type depletion
enhancement layer 8n can be readily prepared.
[0172] In the semiconductor laser device according to this embodiment, an undoped layer
may be formed between the n-type depletion enhancement layer 8n and the p-type cladding
layer 6. Alternatively, an undoped layer may be formed between the n-type depletion
enhancement layer 8n and the first current blocking layer 9 having a low carrier concentration.
In this case, an undoped layer made of a material having a larger band gap than the
first current blocking layer 9 having a low carrier concentration is preferably formed.
In addition, the material for the undoped layer preferably has a band gap smaller
than that of the p-type cladding layer 6 and larger than that of the first current
blocking layer 9 having a low carrier concentration.
(12) Twelfth Embodiment
[0173] A semiconductor laser device according to a twelfth embodiment of the present invention
is now described.
[0174] The structure of the semiconductor laser device according to the twelfth embodiment
is similar to that shown in Fig. 23, except the materials, thicknesses and carrier
concentrations of respective layers. Table 12 shows the materials, thicknesses and
carrier concentrations of the respective layers of the semiconductor laser device
according to this embodiment.

[0175] As shown in Table 12, a n-type depletion enhancement layer 8n formed with n impurity
levels (donor levels) of the opposite conduction type to a p-type cladding layer 6
is formed between the p-type cladding layer 6 and a first current blocking layer 9
having a low carrier concentration in the semiconductor laser device according to
this embodiment. In this embodiment, therefore, carriers supplied from the donor levels
formed on the n-type depletion enhancement layer 8n compensate for those supplied
from the p-type cladding layer 6, similarly to the tenth embodiment. Therefore, the
quantity of carriers stored in the first current blocking layer 9 having a low carrier
concentration is reduced.
[0176] In this case, the band gap of the n-type depletion enhancement layer 8n and that
of the first current blocking layer 9 having a low carrier concentration are equal
to each other, and the band gaps of these layers 8n and 9 are smaller than that of
the p-type cladding layer 6.
[0177] The first current blocking layer 9 having a low carrier concentration is kept in
a depleted state due to reduction of the quantity of carriers stored therein, whereby
electric capacitance generated between the first current blocking layer 9 having a
low carrier concentration and the p-type cladding layer 6 can be reduced for sufficiently
increasing the operating speed of the semiconductor laser device. In this embodiment,
therefore, the high-frequency characteristic of the semiconductor laser device is
improved.
[0178] In the semiconductor laser device according to this embodiment, the thickness t of
the n-type depletion enhancement layer 8n having a carrier concentration of 5 x 10
17 cm
-3 is varied for measuring the cutoff frequency of the semiconductor laser device at
each level of the thickness t. Consequently, it is proved that the cutoff frequency
is remarkably improved when the thickness t of the n-type depletion enhancement layer
8n is 35 nm.
(13) Thirteenth Embodiment
[0179] Fig. 28 is a typical sectional view showing a semiconductor laser device according
to a thirteenth embodiment of the present invention.
[0180] The structure of the semiconductor laser device shown in Fig. 28 is similar to that
of the semiconductor laser device shown in Fig. 23 except the following point:
[0181] In the semiconductor laser device shown in Fig. 28, an intermediate band gap layer
80 having a band gap smaller than that of a p-type cladding layer 6 and larger than
that of a n-type depletion enhancement layer 8n is formed between the p-type cladding
layer 6 and the n-type depletion enhancement layer 8n. Table 13 shows the structure
of the semiconductor laser device shown in Fig. 28.

[0182] As shown in Table 13, the n-type depletion enhancement layer 8n formed with n impurity
levels (donor levels) of the opposite conduction type to the p-type cladding layer
6 is formed between the p-type cladding layer 6 and a first current blocking layer
9 having a low carrier concentration in the semiconductor laser device according to
this embodiment. In this embodiment, therefore, carriers supplied from the donor levels
formed on the n-type depletion enhancement-layer 8n compensate for those supplied
from the p-type cladding layer 6, similarly to the tenth embodiment. Therefore, the
quantity of carriers stored in the first current blocking layer 9 having a low carrier
concentration is reduced.
[0183] The first current blocking layer 9 having a low carrier concentration is kept in
a depleted state due to such reduction of the quantity of carriers stored therein,
whereby electric capacitance generated between the first current blocking layer 9
having a low carrier concentration and the p-type cladding layer 6 can be reduced
for sufficiently increasing the operating speed of the semiconductor laser device.
Thus, the high-frequency characteristic of the semiconductor laser device is improved
in the present invention.
[0184] In this embodiment, the band gap of the n-type depletion enhancement layer 8n and
that of the first current blocking layer 9 having a low carrier concentration are
equal to each other, and the band gaps of these layers 8n and 9 are smaller than that
of the p-type cladding layer 6. Further, the intermediate band gap layer 80 having
the intermediate band gap between the p-type cladding layer 6 and the n-type depletion
enhancement layer 8n is formed between the p-type cladding layer 6 and the n-type
depletion enhancement layer 8n. The embodiment having such an intermediate band gap
layer 80 attains the following effect:
[0185] In this case, the band gaps of the p-type cladding layer 6, the intermediate band
gap layer 8 and the n-type depletion enhancement layer 8n are reduced in this order,
whereby the band offset between the p-type cladding layer 6 and the intermediate band
gap layer 80 is smaller than that between the p-type cladding layer 6 and the n-type
depletion enhancement layer 8n. Thus, carriers are hardly injected from the p-type
cladding layer 6 into the intermediate band gap layer 80, and hardly injected into
the n-type depletion enhancement layer 8n either. In this case, further, carriers
are injected into both of the intermediate band gap layer 80 and the n-type depletion
enhancement layer 8n in a divided manner, whereby the quantity of carriers injected
into the n-type depletion enhancement layer 8n is reduced.
[0186] Thus, the quantities of carriers injected into the n-type depletion enhancement layer
8n as well as those injected into the first current blocking layer 9 having a low
carrier concentration can be reduced by forming the intermediate band gap layer 80.
Therefore, the quantity of carriers stored in the first current blocking layer 9 having
a low carrier concentration can be further reduced.
[0187] In the semiconductor laser device according to this embodiment having the intermediate
band gap layer 80, as hereinabove described, electric capacitance generated between
the first current blocking layer 9 having a low carrier concentration and the p-type
cladding layer 6 can be further reduced, thereby more improving the operating speed
of the semiconductor laser device.
[0188] In the semiconductor laser device according to this embodiment, the thickness t of
the n-type depletion enhancement layer 8n having a carrier concentration of 5 × 10
17 cm
-3 is varied for measuring the cutoff frequency of the semiconductor laser device at
each level of the thickness t. Consequently, it is proved that the cutoff frequency
is remarkably improved when the thickness t of the n-type depletion enhancement layer
8n is in the range of 15 to 35 nm.
[0189] Further, the thickness t of the n-type depletion enhancement layer 8n was set to
30 nm and the carrier concentration thereof is varied for measuring the cutoff frequency
of the semiconductor laser device at each level of the carrier concentration. Consequently,
it is proved that the cutoff frequency is remarkably improved when the carrier concentration
of the n-type depletion enhancement layer 8n is in the range of 2.5 × 10
17 to 6 × 10
17 cm
-3.
[0190] As understood from the aforementioned results, the ranges of the thickness t and
the carrier concentration of the n-type depletion enhancement layer 8n capable of
remarkably improving the cutoff frequency of the semiconductor laser device are widened
in the embodiment provided with the intermediate band gap layer 80 as compared with
the first embodiment having no intermediate band gap layer. Therefore, the thickness
t and the carrier concentration of the n-type depletion enhancement layer 8n can be
readily set and the n-type depletion enhancement layer 8n can be readily prepared.
(14) Fourteenth Embodiment
[0191] Fig. 29 is a typical sectional view of a semiconductor laser device according to
a fourteenth embodiment of the present invention.
[0192] In the semiconductor laser device shown in Fig. 29, respective layers 2 to 5 are
formed on an n-GaAs substrate 1, similarly to the semiconductor laser device shown
in Fig. 23.
[0193] A cladding layer 91 of p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 200 nm is formed on the guide layer 5. The carrier concentration
of the p-type cladding layer 91 is 3 × 10
17 cm
-3.
[0194] A depletion enhancement layer 92n of n-Ga
0.5In
0.5P, a first current blocking layer 93 of undoped GaAs of a low carrier concentration
having a thickness of 1000 nm and a second current blocking layer 94 of n-GaAs having
a thickness of 500 nm are successively formed on the p-type cladding layer 91.
[0195] Central regions of the n-type depletion enhancement layer 92n, the first current
blocking layer 93 having a low carrier concentration layer and the second current
blocking layer 94 are removed to define a striped opening. The carrier concentration
of the n-type depletion enhancement layer 92n is 5 × 10
17 cm
-3. The carrier concentration of the second current blocking layer 94 is 8 × 10
17 cm
-3.
[0196] A cladding layer 95 of p-(Al
0.7Ga
0.3)
0.5In
0.5P having a thickness of 1300 nm is formed on the p-type cladding layer 91 and the
second current blocking layer 94 to fill up the striped opening. The carrier concentration
of the p-type cladding layer 95 is 3 x 10
17 cm
-3. Thus, the semiconductor device shown in Fig. 29 has a self-aligned structure.
[0197] A contact layer 96 of p-Ga
0.5In
0.5P having a thickness of 200 nm is formed on the p-type cladding layer 95. A contact
layer 97 of p-GaAs having a thickness of 3000 nm is formed on the p-type contact layer
96. The carrier concentration of the p-type contact layer 96 is 2 × 10
18 cm
-3. The carrier concentration of the p-type contact layer 97 is 3 × 10
19 cm
-3.
[0198] Table 14 shows the aforementioned structure.

[0199] Figs. 30 and 31 are typical sectional views showing steps in a method of fabricating
the semiconductor laser device shown in Fig. 29.
[0200] As shown in Fig. 30, the cladding layer 2 of n-(Al
0.7Ga
0.3)
0.5In
0.5P, the guide layer 3 of (Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm, the quantum well active layer 4, the guide layer of
(Al
0.5Ga
0.5)
0.5In
0.5P having a thickness of 30 nm, the cladding layer 91 of p-(Al
0.7Ga
0.3)
0.5In
0.5P, the depletion enhancement layer 92n of n-Ga
0.5In
0.5P, the first current blocking layer 93 of undoped GaAs having a low carrier concentration
and the second current blocking layer 94 of n-GaAs are successively grown on the n-GaAs
substrate 1 by MOCVD.
[0201] A mask (not shown) is formed on the second current blocking layer 94 and patterned
to have a striped opening. Thereafter the central portions of the second current blocking
layer 94, the first current blocking layer 93 having a low carrier concentration and
the depletion enhancement layer 92n are removed by etching for forming the striped
opening, as shown in Fig. 31.
[0202] Then, the cladding layer 95 of p-(Al
0.7Ga
0.3)
0.5In
0.5P, the contact layer 96 of p-Ga
0.5In
0.5P and the contact layer 97 of p-GaAs are successively formed on the second current
blocking layer 94 and on the p-type cladding layer 91 located in the striped opening
by MOCVD, as shown in Fig. 29. A p-electrode 12 of Cr/Au is formed on the surface
of the p-type contact layer 97, and an n electrode 13 of AuGe/Ni/Au is formed on the
back side of the n-GaAs substrate 1.
[0203] In the semiconductor laser device shown in Fig. 31, the n-type depletion enhancement
layer 92n formed with impurity levels (donor levels) of the opposite conduction type
to the p-type cladding layer 91 is formed between the p-type cladding layer 91 and
the first current blocking layer 93 having a low carrier concentration. In this embodiment,
therefore, carriers supplied from the donor levels formed on the n-type depletion
enhancement layer 92n compensate for those supplied from the p-type cladding layer
91, similarly to the tenth embodiment. Therefore, the quantity of carriers stored
in the first current blocking layer 93 having a low carrier concentration is reduced.
[0204] The first current blocking layer 93 having a low carrier concentration is kept in
a depleted state due to such reduction of the quantity of carriers stored therein,
whereby electric capacitance generated between the first current blocking layer 93
having a low carrier concentration and the p-type cladding layer 91 can be reduced
for sufficiently increasing the operating speed of the semiconductor laser device.
Thus, the high-frequency characteristic of the semiconductor laser device is improved
in this embodiment.
[0205] In the semiconductor laser device shown in Fig. 29, the thickness t of the n-type
depletion enhancement layer 92n having a carrier concentration of 5 x 10
17 cm
-3 is varied for measuring the cutoff frequency of the semiconductor laser device at
each level of the thickness t. Consequently, it is proved that the cutoff frequency
is remarkably improved when the thickness t of the n-type depletion enhancement layer
92n is in the range of 20 to 35 nm.
[0206] Further, the thickness t of the n-type depletion enhancement layer 92n is set to
30 nm and the carrier concentration thereof is varied for measuring the cutoff frequency
of the semiconductor laser device at each level of the carrier concentration. Consequently,
it is proved that the cutoff frequency is remarkably improved when the carrier concentration
of the n-type depletion enhancement layer 92n is in the range of 3 x 10
17 to 6 x 10
17 cm
-3.
(15) Fifteenth Embodiment
[0207] A semiconductor laser device according to a fifteenth embodiment of the present invention
is now described.
[0208] The structure of the semiconductor laser device according to the fifteenth embodiment
is similar to that shown in Fig. 29, except the materials, thicknesses and carrier
concentrations of respective layers. Table 15 shows the materials, thicknesses and
carrier concentrations of the respective layers of the semiconductor laser device
according to this embodiment.

[0209] As shown in Table 15,an-type depletion enhancement layer 92n formed with impurity
levels (donor levels) of the opposite conduction type to a p-type cladding layer 91
is formed between the p-type cladding layer 91 and a first current blocking layer
93 having a low carrier concentration in the semiconductor laser device according
to this embodiment. In this embodiment, therefore, carriers supplied from the donor
levels formed on the n-type depletion enhancement layer 92n compensate-for those supplied
from the p-type cladding layer 91, similarly to the tenth embodiment. Therefore, the
quantity of carriers stored in the first current blocking layer 93 having a low carrier
concentration is reduced.
[0210] The first current blocking layer 93 having a low carrier concentration is kept in
a depleted state due to such reduction of the quantity of carriers stored therein,
whereby electric capacitance generated between the first current blocking layer 93
having a low carrier concentration and the p-type cladding layer 91 can be reduced
for sufficiently increasing the operating speed of the semiconductor laser device.
Thus, the high-frequency characteristic of the semiconductor laser device is improved
in this embodiment.
[0211] Fig. 32 is a diagram showing the results of measurement of a cutoff frequency of
the semiconductor laser device shown in Table 15 with variation of the thickness t
of the n-type depletion enhancement layer 92n. In this case, the carrier concentration
of the n-type depletion enhancement layer 92n is 5 x 10
17 cm
-3 .
[0212] The cutoff frequency, 200 MHz when the semiconductor laser device is formed with
no n-type depletion enhancement layer 92n, is gradually improved when the thickness
of the n-type depletion enhancement layer 92n is increased, and remarkably improved
when the thickness t of the n-type depletion enhancement layer 92n is in the range
of-15 to 35 nm, as shown in Fig. 32.
[0213] The thickness t of the n-type depletion enhancement layer 92n is set to 25 nm and
the carrier concentration thereof is varied for measuring the cutoff frequency of
the semiconductor laser device at each level of the carrier concentration. Consequently,
it is proved that the cutoff frequency is remarkably improved when the carrier concentration
of the n-type depletion enhancement layer 92n is in the range of 3.5 × 10
17 to 8 × 10
17 cm
-3.
(16) Sixteenth Embodiment
[0214] A semiconductor laser device according to a sixteenth embodiment of the present invention
is now described.
[0215] The structure of the semiconductor laser device according to the sixteenth embodiment
is similar to that shown in Fig. 29, except the materials, thicknesses and carrier
concentrations of respective layers. Table 16 shows the materials, thicknesses and
carrier concentrations of the respective layers of the semiconductor laser device
according to this embodiment.

[0216] As shown in Table 16,an-type depletion enhancement layer 92n formed with impurity
levels (donor levels) of the opposite conduction type to a p-type cladding layer 91
is formed between the p-type cladding layer 91 and a first current blocking layer
93 having a low carrier concentration in the semiconductor laser device according
to this embodiment. In this embodiment, therefore, carriers supplied from the donor
levels formed in the n-type depletion enhancement layer 92n compensate for those supplied
from the p-type cladding layer 91, similarly to the tenth embodiment. Therefore, the
quantity of carriers stored in the first current blocking layer 93 having a low carrier
concentration is reduced.
[0217] The first current blocking layer 93 having a low carrier concentration is kept in
a depleted state due to such reduction of the quantity of carriers stored therein,
whereby electric capacitance generated between the first current blocking layer 93
having a low carrier concentration and the p-type cladding layer 91 can be reduced
for sufficiently increasing the operating speed of the semiconductor laser device.
Thus, the high-frequency characteristic of the semiconductor laser device is improved
in this embodiment.
[0218] According to this embodiment, the semiconductor laser device is so set that the band
gap of the n-type depletion enhancement layer 92n is smaller than that of the p-type
cladding layer 91 and larger than that of the first current blocking layer 93 having
a low carrier concentration. The n-type depletion enhancement layer 92n according
to this embodiment also has the function as the intermediate band gap layer 80 in
the thirteenth embodiment.
[0219] In this case, the band gaps of the p-type cladding layer 91, the n-type depletion
enhancement layer 92n and the first current blocking layer 93 having a low carrier
concentration are reduced in this order. Therefore, the band offset between the p-type
cladding layer 91 and the n-type depletion enhancement layer 92n is smaller than that
between the p-type cladding layer 91 and the first current blocking layer 93 having
a low carrier concentration. Thus, carriers are hardly injected from the p-type cladding
layer 91 into the n-type depletion enhancement layer 92n, and hardly injected into
the first current blocking layer 93 having a low carrier concentration either. In
this case, further, carriers are injected into both of the n-type depletion enhancement
layer 92n and the first current blocking layer 93 having a low carrier concentration
in a divided manner, whereby the quantity of carriers injected into the first current
blocking layer 93 having a low carrier concentration is reduced.
[0220] As hereinabove described, the quantity of carriers stored in the first current blocking
layer 93 having a low carrier concentration can be further reduced by forming the
n-type depletion enhancement layer 92n also serving as the intermediate band gap layer.
[0221] Thus, electric capacitance generated between the first current blocking layer 93
having a low carrier concentration and the p-type cladding layer 91 can be further
reduced in the embodiment having the n-type depletion enhancement layer 92n also serving
as the intermediate band gap layer, whereby the operating speed of the semiconductor
laser device is further improved.
[0222] In the semiconductor laser device according to this embodiment, the thickness t of
the n-type depletion enhancement layer 92n having a carrier concentration of 5 x 10
17 cm
-3 is varied for measuring the cutoff frequency of the semiconductor laser device at
each level of the thickness t. Consequently, it is proved that the cutoff frequency
is remarkably improved when the thickness t of the n-type depletion enhancement layer
92n is 40 nm.
[0223] In the semiconductor laser device according to this embodiment, an undoped layer
may be formed between the n-type depletion enhancement layer 92n and the p-type cladding
layer 91. Alternatively, an undoped layer may be formed between the n-type depletion
enhancement layer 92n and the first current blocking layer 93 having a low carrier
concentration. In this case, an undoped layer made of a material having a larger band
gap than the first current blocking layer 93 having a low carrier concentration is
preferably formed. In addition, the material for the undoped layer preferably has
a band gap smaller than that of the p-type cladding layer 91 and larger than that
of the first current blocking layer 93 having a low carrier concentration.
[0224] In each of the aforementioned tenth to sixteenth embodiments, the donor levels are
formed on the depletion enhancement layer by doping the same with an n-type impurity.
In this case, the donor levels formed on the depletion enhancement layer are preferably
set to such density that most of the donor levels ionize in a state applying no bias
voltage voltage.
[0225] While the n-type depletion enhancement layer of the opposite conduction type is formed
on the region of the p-type cladding layer excluding the current injection region
in each of the aforementioned tenth to sixteenth embodiments, the n-type depletion
enhancement layer may alternatively be formed on the current injection region of the
p-type cladding layer. In this case, however, the thickness of the n-type depletion
enhancement layer is reduced not to inhibit the current injection.
[0226] While the reverse conduction type second current blocking layer is provided on the
first current blocking layer having a low carrier concentration in each of the aforementioned
tenth to sixteenth embodiments, the reverse conduction type second current blocking
layer is not necessarily required in the present invention but only the first current
blocking layer having a low carrier concentration may be formed as the current blocking
layer.
[0227] While the semiconductor laser device is prepared from a group III nitride semiconductor,
an AlGaInP based semiconductor or an AlGaAs based semiconductor in each of the aforementioned
tenth to sixteenth embodiments, the present invention is also applicable to other
group III-V, group II-VI, group IV and group IV-IV semiconductors such as GaInAs.
The present invention is particularly effective for a semiconductor laser device employing
a semiconductor such as a group III nitride semiconductor, an AlGaInP based semiconductor
or an AlGaAs based semiconductor, for example, hardly allowing formation of a semi-insulating
semiconductor during epitaxy.
[0228] The principle and the function of the present invention are now described in detail.
[0229] It is assumed that e represents elementary electric charge, ε
c represents the dielectric constant of a cladding layer, ε represents the dielectric
constant of a depletion enhancement layer, N
c represents the carrier concentration of the cladding layer, N represents the carrier
concentration of the depletion enhancement layer, E
gc represents the band gap of the cladding layer, ΔE
v represents the valence band discontinuity at interface of the cladding layer and
the depletion enhancement layer, and ΔE
c represents the quantity of conduction band discontinuity between the cladding layer
and the depletion enhancement layer.
[0230] Consider that the cladding layer is of the n type and acceptor levels are formed
on the depletion enhancement layer, as shown in Figs. 33(a) and 33(b).
[0231] Also consider that relation ε
cN
cΔE
c/(εN) < ε
gc - ΔE
c - ΔE
v holds while satisfying the following condition (1) or (2):
(1) The cladding layer and the depletion enhancement layer have no distortion.
(2) Each layer is made of a semiconductor having a zinc-blend structure and the stacking
direction of each layer is expressed in the direction of a general formula [0MN] (M
and N represent arbitrary numbers excluding M = N = 0, e.g., the [001] direction and
the [011] direction) or each layer is made of a semiconductor having a wurtzite structure
and the stacking direction of each layer is expressed in the direction of a general
formula [HKL0] direction (H, K and L represent arbitrary numbers satisfying H + K
+ L = 0 excluding H = K = L = 0, e.g., the [1100] direction and the [1120] direction).
[0232] Fig. 33(a) shows the band structures of the cladding layer and the depletion enhancement
layer in the case where the thickness t of the depletion enhancement layer is about
t
A = [2ε
cN
cΔE
c]
1/2/(eN) .
[0233] As shown in Fig. 33(a), the energy of the lowest portion (the interface between the
depletion enhancement layer and the cladding layer) of the conduction band of the
depletion enhancement layer is equal to that of the lowest portion of the conduction
band of the cladding layer. Therefore, no carriers are stored on the interface between
the conduction band of the depletion enhancement layer and that of the gadding layer.
[0234] Fig. 33 (b) shows the band structures of the cladding layer and the depletion enhancement
layer in the case where the thickness t of the depletion enhancement layer is about
t
B = [2ε
cεN
c(E
gc - ΔE
v)/[N(ε
cN
c + εN)]]
1/2/e.
[0235] As shown in Fig. 33(b) , the energy of the highest portion (the interface between
the depletion enhancement layer and a first current blocking layer having a low carrier
concentration) of the valence band of the depletion enhancement layer is equal that
of the lowest portion of the conduction band of the cladding layer. Therefore, all
acceptor levels of the depletion enhancement layer ionize to supply no carriers the
current blocking layer having a low carrier concentration.
[0236] When the thickness t of the depletion enhancement layer is in the range of t
A ≦ t ≦ t
B , the quantity of charges stored on the interface between the cladding layer and
the depletion enhancement layer remains unchanged with respect to small change of
an electric field applied to the interface. In other words, transient electric capacitance
can be approximated to zero on the interface between the cladding layer and the depletion
enhancement layer. Thus, the thickness t of the depletion enhancement layer is preferably
set in the aforementioned range in particular.
[0237] When ε
cN
cΔE
c/ (εN) = E
gc - ΔE
v, t
A = t
B. Therefore, it is difficult to set t = t
A = t
B by adjusting the thickness t and the carrier concentration N of the depletion enhancement
layer. In other words, the thickness t and the carrier concentration N of the depletion
enhancement layer can be more readily set as the value of E
gc - △E
c - △E
v is increased beyond the value of ε
cN
cΔE
c/ (εN) .
[0238] In a general combination of materials for a cladding layer and a depletion enhancement
layer employed for a semiconductor laser device, ε
c is substantially equal to ε, and ΔE
c is less than the band gap (E
gc - ΔE
c - ΔE
v) of the depletion enhancement layer. While ε
cN
cΔE
c/ (εN) may be greater than E
gc - ΔE
v - ΔE
c when N
c » N, the relation ε
cN
cΔE
c/ (εN) < E
gc - ΔE
c - ΔE
v can be readily satisfied when N » N
c or N is substantially equal to N
c.
[0239] While the case were the cladding layer is of the n type and the acceptor levels are
formed on the depletion enhancement layer has been considered in the above discussion,
a similar effect is attained also when the cladding layer is of the p type and the
depletion enhancement layer is formed with donor levels as in the aforementioned embodiments.
[0240] When the relation ε
cN
cΔE
c/ (εN) < E
gc - ΔE
c - ΔE
v holds in the p-type cladding layer and the depletion enhancement layer formed with
donor levels, the quantity of charges stored in the interface between the cladding
layer and the depletion enhancement layer remains unchanged with respect to small
change of an electric field applied to the interface if the thickness t is in the
range of t
A ≦ t ≦ t
B. In other words, transient electric capacitance can be approximated to zero in the
interface between the cladding layer and the -depletion enhancement layer. Therefore,
the thickness t of the depletion enhancement layer formed with the donor levels is
preferably set in the aforementioned range in particular. Further, the carrier concentration
N
c of the p-type cladding layer and the carrier concentration N of the depletion enhancement
layer formed with the donor levels further preferably satisfy N
c ≦ N.
[0241] When forming an intermediate band gap layer between the cladding layer and the depletion
enhancement layer as in the thirteenth embodiment, conditions on the t
A side are widened. When the depletion enhancement layer has the function of the intermediate
band gap layer as in the eleventh and sixteenth embodiments, conditions on the t
A and t
B sides are widened. This appears in the range of the optimum thickness of the depletion
enhancement layer 8n described above with reference to each of the eleventh, thirteenth
and sixteenth embodiments.
[0242] Neither of the conditions (1) and (2) applies to the semiconductor laser device according
to each of the twelfth and sixteenth embodiments but a potential gradient resulting
from a piezoelectric field is caused in the cladding layer due to distortion. Therefore,
the aforementioned relations cannot be readily guided in these embodiments.
[0243] Although the present invention has been described and illustrated in detail, it is
clearly understood that the same is by way of illustration and example only and is
not to be taken by way of limitation, the spirit and scope of the present invention
being limited only by the terms of the appended claims.
1. A semiconductor laser device comprising:
an active layer;
a first cladding layer of a first conduction type provided on said active layer;
a current blocking layer of a second conduction type provided on said first cladding
layer except a current injection region;
a low carrier concentration layer provided on the side of said current blocking layer
between said first cladding layer and said current blocking layer and having a lower
carrier concentration than said current blocking layer; and
a depletion enhancement layer provided on the side of said first cladding layer between
said first cladding layer and said current blocking layer for inhibiting storage of
carriers in said low carrier concentration layer.
2. The semiconductor laser device according to claim 1, wherein
the band gaps of said first cladding layer, said depletion enhancement layer and
said low carrier concentration layer are reduced in this order.
3. The semiconductor laser device according to claim 1, wherein
said first cladding layer has a flat portion formed on said active layer and a ridge
portion formed on said flat portion in said current injection region,
said depletion enhancement layer is formed on said flat portion located on both sides
of said ridge portion and on the side surfaces of said ridge portion, and
said low carrier concentration layer and said current blocking layer are successively
formed on said depletion enhancement layer.
4. The semiconductor laser device according to claim 3, wherein
the thickness of said depletion enhancement layer is at least 10 nm.
5. The semiconductor laser device according to claim 4, wherein
the thickness of said depletion enhancement layer is at least 15 nm.
6. The semiconductor laser device according to claim 1, further comprising a ridge-shaped
second cladding layer of a first conduction type provided on said depletion enhancement
layer in said current injection region, wherein
said depletion enhancement layer is formed on said first cladding layer, and
said lower carrier concentration layer and said current blocking layer are successively
formed on said depletion enhancement layer located on both sides of said second cladding
layer and on the side surfaces of said second cladding layer.
7. The semiconductor laser device according to claim 1, wherein
said depletion enhancement layer, said low carrier concentration layer and said current
blocking layer are successively formed on said first cladding layer except said current
injection region,
said semiconductor laser device further comprising a second cladding layer of a first
conduction type provided to fill up a space enclosed with the side surfaces of said
depletion enhancement layer, said low carrier concentration layer and said current
blocking layer and the upper surface of said first cladding layer in said current
injection region.
8. The semiconductor laser device according to claim 6, wherein
the thickness of said depletion enhancement layer is at least 15 nm.
9. The semiconductor laser device according to claim 8, wherein
the thickness of said depletion enhancement layer is at least 20 nm.
10. The semiconductor laser device according to claim 7, wherein
the thickness of said depletion enhancement layer is at least 15 nm.
11. The semiconductor laser device according to claim 10, wherein
the thickness of said depletion enhancement layer is at least 20 nm.
12. The semiconductor laser device according to claim 1, wherein
said depletion enhancement layer has a single-layer structure or a superlattice
structure.
13. The semiconductor laser device according to claim 1, wherein
said active layer includes a layer made of (Alx1Ga1- x1)y1In1-y1P,
said depletion enhancement layer is made of (Alx2Ga1- x2)y2In1-y2P or Alx2Ga1-x2As,
said low carrier concentration layer is made of (Alx3Ga1-x3)y3In1-y3P or Alx3Ga1-x3As,
said current blocking layer is made of (Alx4Ga1-x4)y4In1-y4P or Alx4Ga1-x4As, and
said x1, said x2, said x3, said x4, said y1, said y2, said y3 and said y4 are at least
zero and not more than 1 respectively.
14. The semiconductor laser device according to claim 1, wherein
said active layer includes a layer made of Alx1Ga1-x1As,
said depletion enhancement layer is made of Alx2Ga1-x2As,
said low carrier concentration layer is made of Alx3Ga1-x3As,
said current blocking layer is made of Alx4Ga1-x4As, and
said x1, said x2, said x3 and said x4 are at least zero and not more than 1 respectively.
15. The semiconductor laser device according to claim 1, wherein
said active layer is made of Inx1Ga1-x1N,
said depletion enhancement layer is made of Alx2Ga1-x2N,
said low carrier concentration layer is made of Alx3Ga1-x3N,
said current blocking layer is made of Alx4Ga1-x4N, and
said x1, said x2, said x3 and said x4 are at least zero and not more than 1 respectively.
16. The semiconductor laser device according to claim 1, wherein
said active layer includes a layer made of (Alx1Ga1- x1)y1In1-y1P,
said depletion enhancement layer is made of (Alx2Ga1- x2)y2In1-y2P ,
said low carrier concentration layer is made of Alx3Ga1-x3As,
said current blocking layer is made of Alx4Ga1-x4As,
said x1, said x2, said x3, said x4, said y1 and said y2 are at least zero and not
more than 1 respectively, and
said first conduction type is the p type, and said second conduction type is the n
type.
17. A semiconductor laser device comprising:
an active layer;
a first cladding layer of a first conduction type provided on said active layer;
a first current blocking layer having a low carrier concentration provided on said
first cladding layer except a current injection region; and
a depletion enhancement layer formed between said first cladding layer and said first
current blocking layer for inhibiting storage of carriers in said first current blocking
layer, wherein
said depletion enhancement layer has an energy level in band gap supplying second
conduction type carriers to compensate for first conduction type carriers supplied
from said first cladding layer due to a modulation doping effect.
18. The semiconductor laser device according to claim 17, wherein
said first current blocking layer has a narrower band gap than said first cladding
layer.
19. The semiconductor laser device according to claim 17, wherein
said energy level in band gap has such density that substantially all said band-to-band
levels ionize under a condition applying no bias voltage.
20. The semiconductor laser device according to claim 17, wherein
said energy level in band gap is formed by doping with a second conduction type
impurity.
21. The semiconductor laser device according to claim 17, wherein
the material of said depletion enhancement layer is the same as the material of
said first current blocking layer.
22. The semiconductor laser device according to claim 17, wherein
said first cladding layer has a larger band gap than said depletion enhancement layer,
said semiconductor laser device further comprising an intermediate band gap layer
provided between said first cladding layer and said depletion enhancement layer and
having a band gap smaller than the band gap of said first cladding layer and larger
than the band gap of said depletion enhancement layer.
23. The semiconductor laser device according to claim 17, wherein
said depletion enhancement layer has a band gap smaller than the band gap of said
first cladding layer and larger than the band gap of said first current blocking layer.
24. The semiconductor laser device according to claim 17, wherein
said first cladding layer has a flat portion formed on said active layer and a ridge
portion formed on said flat portion in said current injection region,
said depletion enhancement layer is formed on-said flat portion located on both sides
of said ridge portion and on the side surfaces of said ridge portion, and
said first current blocking layer is formed on said depletion enhancement layer.
25. The semiconductor laser device according to claim 17, wherein
said depletion enhancement layer and said first current blocking layer are successively
formed on said first cladding layer except said current injection region,
said semiconductor laser device further comprising a second cladding layer of a first
conduction type provided to fill up a space enclosed with the side surfaces of said
depletion enhancement layer and said first current blocking layer and the upper surface
of said first cladding layer in said current injection region.
26. The semiconductor laser device according to claim 17, wherein
said depletion enhancement layer is formed on a region excluding said current injection
region.
27. The semiconductor laser device according to claim 17, further comprising a second
current blocking layer of a second conduction type provided on said first current
blocking layer.
28. A semiconductor laser device comprising:
an active layer;
a first cladding layer of a first conduction type provided on said active layer;
a first current blocking layer having a low carrier concentration provided on said
first cladding layer except a current injection region; and
a depletion enhancement layer formed between said first cladding layer and said first
current blocking layer for inhibiting storage of carriers in said first current blocking
layer.
29. The semiconductor laser device according to claim 28, wherein
said first current blocking layer having a low carrier concentration has a narrower
band gap than said first cladding layer.
30. The semiconductor laser device according to claim 28, wherein
the band gaps of said first cladding layer, said depletion enhancement layer and
said first current blocking layer having a low carrier concentration are reduced in
this order.
31. The semiconductor laser device according to claim 28, wherein
said first cladding layer has a flat portion formed on said active layer and a ridge
portion formed on said flat portion in said current injection region,
said depletion enhancement layer is formed on said flat portion located on both sides
of said ridge portion and on the side surfaces of said ridge portion, and
said first current blocking layer having a low carrier concentration is formed on
said depletion enhancement layer.
32. The semiconductor laser device according to claim 28, further comprising a ridge-shaped
second cladding layer of a first conduction type provided on said depletion enhancement
layer in said current injection region, wherein
said depletion enhancement layer is formed on said first cladding layer, and
said first current blocking layer having a lower carrier concentration is formed on
said depletion enhancement layer located on both sides of said second cladding layer
and on the side surfaces of said second cladding layer.
33. The semiconductor laser device according to claim 28, wherein
said depletion enhancement layer and said first current blocking layer having a low
carrier concentration are successively formed on said first cladding layer except
said current injection region,
said semiconductor laser device further comprising a second cladding layer of a first
conduction type provided to fill up a space enclosed with the side surfaces of said
depletion enhancement layer and said first current blocking layer having a low carrier
concentration and the upper surface of said first cladding layer in said current injection
region.